This invention relates generally to radiological well logging apparatus and methods for investigating subsurface earth formations traversed by a borehole and, more particularly, provides improved methods and apparatus for pulsed neutron/gamma ray logging to identify and measure various specific elements in such subsurface earth formations.
A number of well logging techniques in use come under the classification of pulsed neutron/gamma ray logging. In their basic form, these techniques involve cyclically irradiating subsurface earth formations with pulses or bursts of high energy neutrons and determining the effects on formation constituent: of the irradiating neutrons by measuring gamma rays produced thereby. The present invention contemplates the detection and measurement of gamma rays resulting from thermal neutron capture during the interval between source bursts. The energies of the capture gamma rays are characteristic of the specific elements capturing the neutrons, and gamma rays of these characteristic energies are detected to identify and measure these elements.
A primary objective of the present invention is to determine accurately the abundance in subsurface earth formations of particular preselected elements which may be present only in minor proportions. Moreover, the invention is directed at the difficult case where an element of interest is present in both the earth formation and the borehole environment. This circumstance results in spurious or inaccurate results when methods of the prior art are employed, for example, in detecting the presence of low concentrations of salt water in a formation surrounding a salt water injection well, where gamma rays are produced due to capture of neutrons by sodium and chlorine in both the borehole and the formation. It is of course impossible to differentiate between the respective regions of origin of the gamma rays. As a consequence, the small number of gamma rays produced by NaCl of low concentration in the formation will be obscured by interference from the gamma rays produced by the salt water in the borehole. Techniques have been developed to alleviate this problem; however, they often involve multiple measurements or cumbersome and complex corrections and correlations which in themselves may have dubious accuracy.
These and other disadvantages of the prior art are overcome by the present invention which provides methods for directly and more accurately measuring subsurface formation characteristics such as the presence of a particular element or the ratio of the abundance of two particular elements within an earth formation surrounding a borehole.